PROTOZOA. FORAMINIFERA. 15 



means of fission and by the formation of spores. But in 

 some cases conjugation occurs, representing to some extent 

 the sexual method of the higher animals. In some of the 

 Protozoa there is no skeleton, but in others a shell, which 

 may be calcareous, siliceous, or chitinous, is secreted. 



The Protozoa can be divided into three classes, (1) the 

 Rhizopoda, (2) the Infusoria, (3) the Sporozoa or Grega- 

 rinida. The Rliizopods are the only forms which have 

 been definitely recognised in the fossil state. 



CLASS. RHIZOPODA. 



Protozoa which possess no cortical layer and which are 

 able to throw out pseudopodia. 



The Rhizopoda are divided into five or more orders, 

 but only two of these have been definitely found fossil, 

 namely, the Foraminifera (or Reticularia) and the Radio- 

 laria. 



ORDER. FORAMINIFERA. 



The Foraminifera are characterized by giving out 

 filamentous pseudopodia, which frequently branch and 

 anastomose, and by- possessing in most cases a shell or 

 test, which may be calcareous, arenaceous, siliceous, or 

 chitinous. 



The calcareous forms are by far the commonest, and 

 in these two kinds of test may be distinguished, namely, 

 the vitreous and the porcellanous. In the vitreous, the 

 test is glassy and transparent, and is perforated by in- 

 numerable tubes for the passage of the pseudopodia; in 

 some forms {e.g. Rotaliti) these tubes are as much as 

 gifaf of an inch in diameter, but in others (e.g. Operculina) 

 they are only 16 ^ 00 of an inch. In the porcellanous forms, 

 the test, when viewed by reflected light, is opaque and 

 white, having the appearance of porcelain; it is perfectly 



